A gate drive circuit of a voltage-driven semiconductor switching element (a drive target semiconductor element) includes a drive-on element that applies an on-state voltage to a gate of the drive target semiconductor element, and a drive-off element that applies an off-state voltage to the gate. The gate of the drive target semiconductor element is controlled by complementary switching of the drive-on and drive-off elements.
When voltage is applied to the gate of the drive target semiconductor element, a gate voltage changes by a ratio in accordance with a constant determined by a parasitic capacitance of the gate and the wiring resistance connected to the gate. Because the resistance is connected to the gate, the peak current and charge rate must be reduced to mitigate switching noise.
However, there is a problem with conductivity loss caused by the gate resistance due to an increased switching frequency of the drive target semiconductor element, because of which, furthermore, there is a proposal for a still more easily controlled gate drive circuit such that both switching loss and noise are reduced, in addition to which conduction loss in the gate drive circuit is reduced, by the gate resistance being replaced with a reactor, and an LC resonant circuit being configured of the reactor and the parasitic capacitance of the drive target semiconductor element gate as an auxiliary drive unit (Patent Document 1).
That is, the gate drive circuit proposed in Patent Document 1 includes a direct current power supply circuit with an IGBT (insulated gate bipolar transistor) as a semiconductor switching element (a drive target semiconductor element), and a first direct current power supply and a second direct current power supply connected in series with a connection point thereof taken as a power supply intermediate point and adopted as a reference potential, and a drive element unit formed of a drive-on element, which interrupts a first power supply path along which a positive voltage of the first direct current power supply is supplied to a gate of the drive target semiconductor element, and a drive-off element, which interrupts a second power supply path along which a negative voltage of the second direct current power supply is supplied to the gate of the drive target semiconductor element, and furthermore, an auxiliary drive unit wherein a reactor and the switching element are connected in series is provided between the power supply intermediate point and the gate of the drive target semiconductor element.
Further, the drive element unit is controlled by a drive control unit, an on-state voltage necessary in order to achieve an on-state is applied via the drive-on element to the gate of the drive target semiconductor element when the drive control unit turns on the drive-on element, and an off-state voltage necessary in order to achieve an off-state is applied to the gate of the drive target semiconductor element when the drive control unit turns on the drive-off element. Also, a configuration is such that when the drive control unit turns off both the drive-on element and the drive-off element, a resonant circuit is formed of the reactor configuring the auxiliary drive unit and parasitic capacitance of the gate of the drive target semiconductor element.
In a state wherein the gate of the drive target semiconductor element is in an on-state, that is, in a state wherein the drive-on element is turned on and the drive-off element is turned off, current flows from the gate side of the drive target semiconductor element toward the reactor in the reactor. When the drive-on element is turned off in this state, an accumulated charge of the parasitic capacitance of the gate of the drive target semiconductor element is released and becomes zero due to resonance of the resonant circuit, or the current continues to flow so that the parasitic capacitance is further charged at reverse polarity. As a result of this, the gate voltage decreases sharply, together with which voltage across the drive target semiconductor element (source-to drain voltage, collector-to-emitter voltage) increases sharply, and the drive element is turned off. Further, when the gate voltage of the drive target semiconductor element reaches the off-state voltage and the drive control unit turns on the drive-off element, the drive target semiconductor element is held in the off-state by the gate voltage being held at the off-state voltage.
Meanwhile, in a state wherein the gate of the drive target semiconductor element is in an off-state, that is, in a state wherein the drive-on element is turned off and the drive-off element is turned on, current flows from the reactor toward the gate side of the drive target semiconductor element. When the drive-off element is turned off in this state, an accumulated charge of the parasitic capacitance of the gate of the drive target semiconductor element is released, and the parasitic capacitance is further charged at reverse polarity, due to resonance of the resonant circuit, or the current continues to flow in a direction such as to charge from a state wherein the accumulated charge is zero. As a result of this, the gate voltage increases sharply, together with which voltage across the drive target semiconductor element decreases sharply, and the drive target semiconductor element is turned on. Further, when the gate voltage of the drive target semiconductor element reaches the on-state voltage and the drive control unit turns on the drive-on element, the drive target semiconductor element is held in the on-state by the gate voltage being held at the on-state voltage.
In the auxiliary drive unit, current of a magnitude necessary in order to cause the gate voltage to change to the off-state voltage or the on-state voltage due to resonance needs to be caused to flow into the reactor until the drive-on element is turned off when turning off the drive target semiconductor element, and until the drive-off element is turned off when turning on the drive target semiconductor element. In other words, as there is no need to cause a current greater than this to flow, power consumption in the gate drive circuit can be further reduced by controlling a period for which current is caused to flow into the reactor.